Usually, crushing machines are employed to crush target materials to be crushed, e.g., rocks and construction wastes of various sizes generated in construction sites, into a predetermined size for the purposes of reuse of the wastes, smoother progress of work, a cost reduction, etc.
As one example of those crushing machines, a mobile crusher generally comprises a travel body having left and right crawler belts, a crushing device for crushing target materials loaded through a hopper into a predetermined size, a feeder for guiding the target materials loaded through the hopper to the crushing device, a discharge conveyor for carrying the materials having been crushed into small fragments by the crushing device to the outside of the machine, and auxiliaries for performing work related to crushing work performed by the crushing device, such as a magnetic separating device disposed above the discharge conveyor for magnetically attracting and removing magnetic substances included in the crushed materials under carrying on the discharge conveyor.
As disclosed in JP,A 11-226444, for example, a typical hydraulic system for such a self-propelled crushing machine comprises variable displacement hydraulic pumps (i.e., a hydraulic pump for the crushing device and a hydraulic pump for the auxiliaries) driven by a prime mover (engine), a crushing device hydraulic motor and auxiliary hydraulic actuators (such as a feeder hydraulic motor, a discharge conveyor hydraulic motor, and a magnetic separating device hydraulic motor) driven by hydraulic fluids delivered from the hydraulic pumps, a plurality of control valves for controlling the directions and flow rates of the hydraulic fluids supplied from the hydraulic pumps to those hydraulic motors, control means for controlling respective delivery rates of the hydraulic pumps, and so on.
In the known hydraulic drive system, however, when a heavy load is imposed on the crushing device during the crushing work due to, e.g., excessive supply of the target materials (materials to be crushed), a corresponding load is also imposed on the crushing device hydraulic motor and hence the rotational speed of the crushing device hydraulic motor is reduced. This has resulted in problems that crushing efficiency of the crushing device reduces and productivity of crushed products lowers.